The present invention relates to lubricant blends used with refrigerants. More particularly, the present invention relates to lubricants for use with tetrafluoroethane, and preferably, 1,1,1,2-tetrafluoroethane (known in the art as R134a). R134a is a refrigerant which may replace dichlorodifluoromethane (known in the art as R12) in many applications because environmental concerns over the use of R12 exist.
R134a has been mentioned as a possible replacement for R12 because concern over potential depletion of the ozone layer exists. R12 is used in closed loop refrigeration systems; many of these systems are automotive air-conditioning systems. R134a has properties similar to those of R12 so that it is possible to substitute R134a for R12 with minimal changes in equipment being required. The symmetrical isomer of R134a is 1,1,2,2-tetrafluoroethane (known in the art as R134); the isomer is similar in properties also and may also be used. Consequently, it should be understood that in the following discussion, "tetrafluoroethane" will refer to both R134 and R134a.
A unique problem arises in such a substitution. Refrigeration systems which use R-12 generally use mineral oils to lubricate the compressor; (the present discussion does not apply to absorption refrigeration equipment). See for example the discussion in Chapter 32 of the 1980 ASHRAE Systems Handbook. R-12 is completely miscible with such oils throughout the entire range of refrigeration system temperatures, which may range from about -45.6.degree. to 65.6.degree. C. Consequently, oil which dissolves in the refrigerant travels around the refrigeration loop and generally returns with the refrigerant to the compressor. The oil does not separate during condensation, although it may accumulate because low temperatures exist when the refrigerant is evaporated. At the same time, the oil which lubricates the compressor contains some refrigerant which may affect its lubricating property.
R134a is not miscible with mineral oils; consequently, different lubricants will be required for use with R134a. However, as mentioned above, no changes to equipment should be necessary when the refrigerant substitution is made. If the lubricant separates from the refrigerant, it is expected that serious operating problems could result. For example, the compressor could be inadequately lubricated if refrigerant replaces the lubricant. Significant problems in other equipment also could result if a lubricant phase separates from the refrigerant during condensation, expansion, or evaporation. These problems are expected to be most serious in automotive air-conditioning systems because the compressors are not separately lubricated and a mixture of refrigerant and lubricant circulates throughout the entire system.
These problems have been recognized generally in the refrigeration art. Two recent publications by ASHRAE suggest that separation of lubricants and refrigerants presents problems, although no mention is made of R134a. These articles are Kruse et al., "Fundamentals of Lubrication in Refrigeration Systems and Heat Pumps," ASHRAE TRANSACTIONS 90(2B), 763 (1984) and Spauschus, "Evaluation of Lubricants for Refrigeration and Air-Conditioning Compressors," ibid, 784.
The following discussion will be more readily understood if the mutual solubility of refrigerants and various lubricating oils is considered in general, with specific reference to R134a. Small amounts of lubricants may be soluble in R134a over a wide range of temperatures, but as the concentration of the lubricant increases, the temperature range over which complete miscibility occurs, i.e., only one liquid phase is present, narrows substantially. For any composition, two consolute temperatures, i.e., a lower and a higher temperature may exist. That is, a relatively low temperature below which two distinct liquid phases are present and above which the two phases become miscible and a higher temperature at which the single phase disappears and two phases appear again may exist. A diagram of such a system for R502 refrigerant is shown as FIG. 2 in the Kruse et al. paper mentioned above. A range of temperatures where one phase is present exists and while it would be desirable that a refrigeration system operate within such a range, it has been found that for typical compositions, the miscible range of lubricants with R134a is not wide enough to encompass the typical refrigeration temperatures.
Some disclosures which are concerned with the choice of lubricants when R134a is used as a refrigerant exist. Polyalkylene glycols were suggested to be used in Research Disclosure 17463, October 1978 by DuPont. Specific reference was made to such oils produced by Union Carbide Corporation under the trade names "ULCON" (sic) LB-165 and UCON 525. It is stated that these oils are miscible in all proportions with R134a at temperatures at least as low as -50.degree. C. It is believed that "ULCON" (sic) LB-165 and UCON 524 are polyoxypropylene glycols which have a hydroxy group at one end of each molecule and a n-butyl group at the other end.
U.S. Pat. No. 4,302,343 teaches synthetic lubricants comprising esters of hindered polyhydric alcohols with alkanoic acids and polyether polyols; the reference teaches that the lubricants are useful in reciprocating air compressors. Related U.S. Pat. No. 4,751,012 teaches synthetic lubricants comprising esters of monohydric alcohols with aromatic or alkane dicarboxylic acids and polyether polyols.
U.S. Pat. No. 4,428,854 discloses the use of R134a as an absorption refrigerant where organic solvents are used as absorbing agents Tetramethylene glycol dimethyl ether, which is mentioned and has a low molecular weight, is not expected to be useful as a lubricant in compression refrigeration systems.
A related patent, U.S. Pat. No. 4,454,052, also discloses polyethylene glycol methyl ether used as an absorbent along with certain stabilizing materials.
German Unexamined Pat. No. Application 27 50 980 dated May 17, 1979 describes lubricants for refrigeration machines and addresses the problems of suitable lubrication at low-temperatures. The lubricants are fluorosiloxanes. Chlorotrifluoromethane (known in the art as R13) is the only refrigerant mentioned.
U.S. Pat. No. 4,267,064 also recommends the use of polyglycol oils, particularly for rotary compressors. It is indicated that viscosities in the range of 25-50 centistokes (CS) at 98.9.degree. C. are needed plus a viscosity index greater than 150. Many refrigerants are mentioned but not tetrafluoroethane.
Japanese published application No. 51795 of 1982 relates to antioxidants and corrosion inhibitors for use with various polyether type synthetic oils. The tests were carried out with R-12, which does not exhibit the immiscible character of R134a.
U.S. Pat. No. 4,431,557 relates to additives used in synthetic oils. Many refrigerants are mentioned, but not tetrafluoroethane, and the patentees gave no indication of concern with miscibility of the refrigerants and the lubricants.
Japanese Patent Publication No. 179699 dated Oct. 12, 1984 teaches a lubricating oil comprising a diol. The object of the reference is to provide a lubricating oil which does not dissolve refrigerants so as to improve the compressor efficiency, with a high degree of solubility with the refrigerant being undesirable. The reference teaches that the lubrication oil is useful in refrigerators and air conditioners with R12 and monochlorodifluoromethane (known in the art as R22). Many refrigerants are mentioned but not tetrafluoroethane.
Japanese Patent Publication No. 96684 dated May 30, 1985 addresses the stability problems of refrigerants. The reference mentions chlorotrifluoroethylene low-polymer as a useful lubricant. The reference does not address the miscibility problems with R134a.
Commonly assigned U.S. Pat. No. 4,755,316 teaches a compression refrigeration composition. The refrigerant is tetrafluoroethane while the lubricant is at least one polyoxyalkylene glycol which is at least difunctional with respect to hydroxyl groups, has a molecular weight between 300 and 2,000, has a viscosity of about 25-150 centistokes at 37.degree. C., has a viscosity index of at least 20, and is miscible in combination with the tetrafluoroethane in the range between -40.degree. C. and at least +20.degree. C. The reference mentions the use of additives such as viscosity adjusters including polyisobutylene, polymethacrylates, polyalkylstyrenes, naphthenic oils, alkylbenzene oils, paraffinic oils, polyesters, polyvinylchloride, and polyphosphates. The reference states that other types of high viscosity substances could be added to a low molecular weight polyoxypropylene glycol to increase the viscosity provided that the resulting mixtures had a suitable range of miscibility with the refrigerant. The reference does not teach the present composition.
Although the lubricants of commonly assigned U.S. Pat. No. 4,755,316 are miscible with R134a over a wide temperature range, it is desired to have an improved lubricant system. Such a lubricant system might be in the form of a blend of two known lubricants so as to benefit from the properties of each individual component. One requirement of such a blend is that the two lubricants form a single phase because if two phases resulted, distribution of the lubricant's components would be uneven in the various compressor parts. In this regard, we tried combining various known lubricants in order to improve the miscibility range and found that many combinations did not form a single phase.
Because it is expected that R134a will become widely used in the field of refrigeration and air-conditioning, improved lubricants which exist as a single phase and are useful with R134a over a wide miscibility range are needed in the art.